This invention relates to capacitor apparatus in which individual capacitor units are each provided with a discharge damping device for high frequency discharges.
The invention has particular application to power capacitors, such as those which must sustain voltage levels in excess of 1000 volts in normal operation, such as are used for energy storage, power factor correction and series capacitor protection. The invention is particularly beneficial in such capacitors which have internal fuses.
Capacitors have a high efficiency which allows discharge and inrush currents caused by capacitors short circuiting and energization to persist for a time determined by circuit constants. The integral of the discharge current squared over time (.intg..sub.o.sup..infin. i.sup.2 dt=I.sup.2 t) is a parameter used to describe the current's magnitude and duration.
In capacitor installations, it is known to use a damping impedance resistor in the discharge circuit to reduce the current duration and, hence, I.sup.2 t. This has been done in capacitor apparatus in which lumped reactor-resistor combinations are used in the discharge circuit of an entire set or bank of capacitors. A drawback of such arrangements of lumped impedances is that an instance of a bushing flashover or bus flashover, as may affect an individual capacitor unit, but not an entire bank, will not be effectively dampened.
In the context of the present invention, an individual capacitor unit is one can or housing containing a plurality of capacitor sections that are interconnected in a manner to provide a given capacitance at the terminals which extend through the housing. In practice, a plurality of such units are interconnected to provide the bank or capacitor installation. The sections in a capacitor unit are normally connected in a series-parallel configuration. The functioning of an individual section of a unit, which may contain a number such as 20 or so sections, is not always vital to the operation of the unit because of paralleling of the sections. Therefore, it is sometimes the case that the individual sections in a unit have individual fuses. That is, the connections made to an individual winding section include a fusible element that will open upon an excess current rather than having an external fuse for an entire unit. When employing internal fuses, it is necessary that they be large enough, i.e., have enough I.sup.2 t capacity, such that they not melt or fail by fatigue when exposed to capacitor self-discharge currents as would be contemplated to occur occasionally in operation and which are not inherently harmful to the capacitive demands on the unit. Industry standards even require as a test the charging of the capacitor unit to 2.5 times the rated voltage (DC) and discharging into a minimum impedance circuit several times with no fuse operations. A commercially acceptable product must pass such tests. The internal fuses must be capable of withstanding such test and also have a low enough I.sup.2 t capability for voltage interruption in instances in which the dielectric materials of the particular section may be damaged during an interruption. These criteria impose severe restrictions on the proper selection of internal fuses and an object of the present invention is to permit an internally fused unit to be able to employ smaller fuses, with smaller I.sup.2 t capability, while being less subject to fuse operations on discharges of relatively high frequency.
The approach taken by the present invention is to minimize the I.sup.2 t of discharge and inrush currents by a damping device that minimizes the time during which such currents endure and to do so within each individual capacitor unit.
The discharge damping device in accordance with the present invention is preferably installed internal to the capacitor unit so it requires no assembly in the field and is not susceptible to damage in the field. The device basically consists of a series connected resistor paralleled by an inductance shunt connected between the stack of capacitor sections in the unit and one of the terminals extending through the housing. Proper selection of the impedances employed accomplishes a reduction in I.sup.2 t during high frequency discharge which reduces the duty on the capacitor and fuses, whether internal or external, and permits the use of smaller size fuses. This allowable reduction of fuse size is especially important and desirable for internally fused units. The invention further relates to the proper selection of the impedances and an economical and effective physical arrangement of such elements within each individual capacitor unit housing as may be employed in a capacitor installation. Preferably, the invention uses merely two conductors in the discharge damping device whose length and composition determines their resistance and inductance and which are connected in a parallel configuration to produce effective high frequency damping while introducing no appreciable losses at normal operating frequency.